This applications claims the priority of international patent application number PCT/EP98/07175, filed Nov. 10, 1998, and of German patent application number 197 49 688.1, filed Nov. 10, 1997, both of which are incorporated herein by reference.
The invention is related to a method for the combustion of fuels, in which the fuels are burnt together with air, possibly with the addition of water and/or an oxidizing agent, and a reactor for such a combustion method with a reaction chamber having supply openings for the fuel, the air, possibly the water and/or an oxidizing agent and an outlet opening for the combustion products.
An apparatus and a method for the combustion of oil with the addition of water are known of WO95/23942, in which oil is introduced into a combustion chamber until an oil bath has formed, which is then preheated to a temperature between 250xc2x0 C. and 350xc2x0 C. Then water is sprayed onto the surface of the hot oil bath, which results in a flame eruption with the simultaneous supply of air into the combustion chamber. The level of the oil bath should not remain under a height of 3 to 4 mm during combustion in order to prevent an interruption of the combustion. The apparatus used to this purpose includes in general a combustion chamber in the form of a frustrum of a pyramid or a cone with lateral supply openings for oil and water from corresponding reservoirs. The oil bath is electrically heated. Air enters along with the water into the interior of the combustion chamber. The flame with a temperature of 1200xc2x0 C. to 2000xc2x0 C. is introduced into an oven via a cylindrical tube for heating purposes.
In this known method of combustion especially of waste oils the temperature gradient appearing in the oil bath in the direction to the bottom has proved to be disadvantageous, because the bottom temperature can be lower than the evaporation temperatures of heavy fractions in the waste oil the result of which is that the latter form a not completely burnable oil mass at the bottom of the combustion chamber. Injecting the oil via a nozzle is not practical, because residues and highly viscous components in the waste oil will lead to a clogging of the nozzles. Moreover the entire apparatus with its feeding and preheating means gets constructively complex. Because of the remaining residues the process control is hard to perform, especially when shutting down. Therefore the facility is not suited for a continuous operation.
From GB 765 197 an apparatus for the combustion of liquid and liquefiable fuels is known, which consists of a cylindrical combustion chamber with an adjacent fire space, which is open to the top. The liquid fuel is radially or tangentially introduced into the interior of the combustion chamber, and air is separately introduced tangentially, while the fuel is contacting the inner surface of the combustion chamber and is evaporated and burnt there. Temperatures appearing in the fire space are between 1500xc2x0 C. and 1800xc2x0 C. With incomplete combustion by reduced air supply the fuel is cracked with the aid of supplied vapour, whereby heavy oils are decomposed into lower hydrocarbons, hydrogen and carbon monoxide.
Also in this known combustion method the way of supply is technically demanding, and moreover the danger exists that in certain wall regions the temperature is not sufficient for evaporation of heavier waste oil fractions, which then gather at the bottom of the combustion chamber and form a non-burnable residue there. Water vapour is here not provided for the actual combustion but only for cracking of heavy oils.
In U.S. Pat. No. 4,069,005 the combustion of a water/fuel/air mixture in the presence of a metal catalyst (nickel) is proposed, wherein in the interior of the burner several stacked plates, which may also consist of the metal catalyst, can be disposed, to increase the efficiency of the resulting cracking. In the apparatus serving this purpose liquid fuels and water are respectively dropped upon the catalyst from above, the plates having been heated to a temperature above 800xc2x0 C. in a preheating phase. The rising vapours are led along the metal catalysts, whereby easily burnable, gaseous hydrocarbons are generated by cracking, which burn in the further process, whereby combustion gases of 800xc2x0 C. to 1000xc2x0 C. are generated.
For the generation of a long flame for heating an industrial boiler in U.S. Pat. No. 3,804,579 oil and air are burnt together with water vapour, which is generated in a heat exchange coil by the flame. Here the extended flame burns at about 730xc2x0 C.
Finally from DE 39 29 759 C2 a facility for burning waste oil products is known, in which the waste oils are mixed with a usual heating oil with a known smaller viscosity, such that an average product with constant viscosity is, formed, which is then preheated and injected into a tank. On the opposite side of the tank input devices for air, water and common neutralizing agents are provided. For injecting the oil mixture air or water vapour is used. The control facility for the mixing ratio of the oils and the injection apparatus for the oil mixture with additional supply leads for air and neutralizing agents lead to a constructively complex facility, which is hard to control, and which cannot work efficiently, because apart from the actual combustion product of waste oil considerable amounts of normal heating oil have, to be burned additionally, which largely limits the disposal capacity. The simple combustion tank cannot support the combustion process.
It is an object of the present invention to provide a method for the environmentally friendly combustion of fuels of an arbitrary state of aggregation, possibly with the addition of water and/or an oxidizing agent, in which the fuel is burnt without residues with a high energy efficiency. The reactor suitable for this is intended to optimize the combustion process in continuous operation with a low constructive effort, and it should be as maintenance-free as possible, and it should be self-cleaning.
According to the invention the solid and/or liquid and/or gaseous fuel, possibly the water and/or an oxidizing agent are introduced into a reaction chamber under high pressure in axial direction by pressurized air, the amount of injected pressurized air corresponding to the amount of air, which is necessary for the complete combustion, the introduced mixture is led to a deflection surface in the interior of the reaction chamber, whereby it is further atomized, liquid components evaporate, solid ones sublimate and the mixture burns explosively, before it can reach the wall of the bottom of the reaction chamber. The explosive combustion process can be explained by the high degree of the surface increase of the mixture introduced into the reaction chamber:
(a) the fuel supplied by pressurized air is disintegrated and atomized, when it is injected into the reaction chamber,
(b) the existing pressure is still sufficient to lead the fuel with high velocity to a deflection surface in the interior of the reaction chamber, where an impingement and a reflection with a further distribution and atomization are caused.
Additional water injected with pressurized air is atomized into droplets, when entering the reaction chamber, the droplets changing into water vapour and being distributed into all directions in the interior space of the reaction chamber by the deflection surface. The expansion caused by the sudden evaporation supports a mixing of the fuels with the present pressurized air and the water vapour, which leads to an efficient combustion, especially of hardly burnable fuel components. This way a precipitation of fuel at the inner wall and a concentration of residues at the bottom can be more efficiently abided, so that the reactor cleans itself.
The pressurized air flow can be injected at 2 to 10 bar, preferably at 3 to 5 bar into the reaction chamber. At these pressures the combination of the atomization at the exit from the supply lead with the one caused by the impact onto the deflection surface in the interior space of the reaction chamber is especially efficient.
The fuels, the water and/or the oxidizing agent are respectively introduced separately or as a mixture via one or several Venturi tubes into the pressurized air flow. Gaseous fuel can thereby be individually introduced into the reaction chamber. This way of supply allows for a good dosibility with a low constructive effort and simultaneously enhances the atomizing effect at the entrance into the reaction chamber. The injection into the reaction chamber is accomplished by a normal tube of a small diameter without a nozzle top, whereby a clogging of the nozzle at the time of combustion of waste oils by non-burnable residues or highly viscous components is prevented. The constructive effort is lowered furthermore by the use of uniform Venturi tubes for the supply of the fuels and the water.
It is favorable to keep the temperature inside the reaction chamber homogeneous to the axis of the reaction chamber by heat conducting reactor walls. When by the deflection surface a symmetric distribution of the mixture inside, the reaction chamber is caused, a more uniform combustion can be achieved at a symmetric temperature distribution.
At a predetermined geometry of the reaction chamber the inflow velocities into the reaction chamber of the mixture to be burnt can be adjusted, so that the resulting combustion flame leaves the reaction chamber at least with the velocity of sound and the resulting heat energy is transported to the outside for further use. This can be further improved by suitable reactor geometries as described below.
The ignition of the mixture in the reaction chamber is preferably performed by a starter flame or by a generated spark. It can be advantageous to preheat the fuels, the water or the air by the waste heat generated in the combustion, before they are introduced into the reaction chamber. Especially heavy oil becomes easier transportable by the decrease in velocity achieved hereby. The fluid dynamics of the combustion process can be influenced by inserts, that can be introduced into the inner space of the reaction chamber.
It is advantageous to additionally crack the fuel at the time of combustion, wherein as catalyst e.g. a nickel containing material can be used.
The reactor according to the invention has a hyperboloid reactor heads which is adjacent to the outlet opening of the reaction chamber and the cross section of which increases from there. The combustion flame burns at this reactor head. The nozzle like geometry of the reactor thereby causes an acceleration of the combustion gases with the formation of a corresponding vacuum in the outlet region of the reaction chamber, which leads to a further acceleration of the substances to be burnt in the interior of the reaction chamber in the direction of the outlet opening, which positively influences the combustion and the self-cleaning of the reactor.
The nozzle effect can be improved by a tapering of the reaction chamber at least in its upper part in the direction of the outlet opening, whereby the tapering part can be provided specially as a frustrum of a pyramid or a cone. On the other hand the entire reaction chamber can have a hyperboloid shape, so that it tapers in the direction of the outlet opening.
With the nozzle-shaped reactor geometry it is favorable to embed the supply openings for the fuels (and the water) into the bottom of the reaction chamber, so that these are directed parallel to the axis of the reaction chamber. Hereby the axis of the reaction chamber is determined as the preferred flow direction, in which for the better distribution of the mixture to be burnt, a deflection surface can be disposed, by which the mixture is first deflected from the axis of the reaction chamber and is subsequently directed again to this axis by the mentioned nozzle effect. Moreover, the effusion from the supply openings is favored by the pressure conditions.
A cone, the tip of which is directed against the flow direction of the fuel or a pyramid of a fire resistant material, which is directed in the same way, being disposed in the interior of the reaction chamber along its axis, can be used as deflection surface for achieving a homogeneous distribution. The combustion process can thereby be optimized by symmetric distribution in the cross-section of the reaction chamber of physical quantities such as pressure, flow velocity, turbulence and temperature.
If the fuel is intended to be additionally cracked, it is advantageous to provide a metal catalyst, specially a nickel-containing one, e.g. in the interior walls of the reaction chamber in fire-resistant inserts in the interior of the reaction chamber or even in the deflection surface. A high efficiency of the catalytic cracking can be achieved by a scaled or porous metal catalyst with a large surface.
The reactor can uniformly be fabricated of a material like stainless steel, but it can also, at least partially, be fabricated of a specially heat-resistant and mechanically robust alloy like a Nixe2x80x94Moxe2x80x94Crxe2x80x94Co alloy (xe2x80x9cNimonicxe2x80x9d). Moreover, the reactor can be surrounded by an outer insulation of ceramics fibres or fibreglass to reduce the amount of radiated heat and to maintain the temperature in the reaction chamber above 1000xc2x0 C.